This renewal describes experiments using the mammary gland as a model tissue for study of the role of the hypoxic response transcription factor, HIF-1, in growth, differentiation and transformation. It is clear that HIF is a negative factor in human breast cancer progression and that its presence in primary tumors indicates a poor prognosis for this disease. Thus, a better understanding of its role in progression has clear relevance, not just to basic understanding of hypoxic response and malignancy, but to clinical application in women with breast cancer. In this work on HIF and malignancy, we propose to use the insights gained and the reagents generated in the last period of funding to study hypoxic responses of malignant tissue, with an emphasis on their interactions with other tissues during metastatic progression. Metastasis is a central problem in cancer; in some respects, treatment and prevention of metastatic disease is the most important goal in cancer therapy. We have found that metastases are significantly reduced when malignant mammary tissue lacks HIF-1a. This finding demonstrates that this transcriptional pathway plays a key role in allowing the malignant cell to undergo the process of metastasis, and implies an intersection between the events of metastasis and those of hypoxic response. In another unexpected finding, we have shown that loss of HIF-1a in the endothelial cell results in a large reduction of metastasis in vivo. These two findings represent the foundations of the three specific aims that we are proposing for the third period of support for this grant. The specific aims of this proposal are: Specific aim 1: Determine the role of HIF response during malignant epithelial adhesion and transcytosis. Specific Aim 2: Determine the role of HIF-1a in the metastasis of mammary carcinomas. Specific aim 3: Determine the role of epithelial VEGF expression during mammary tumorigenesis and metastasis. PUBLIC HEALTH RELEVANCE: Metastasis is a central problem in cancer; in some respects, treatment and prevention of metastatic disease is the most important goal in cancer therapy. We have found that metastases are significantly reduced when malignant tissue lacks HIF-1a. As we have found important roles for HIF and hypoxic response in tumorigenesis, here we believe that the next frontier lies in understanding the terminal stage of tumorigenic progression: the metastatic event.